Wear-resistant titanium and titanium alloys and method for producing same

ABSTRACT

PRODUCING A TITANIUM OR TITANIUM ALLOY BASE METAL ARTICLE HAVING A HARD SURFACE REGION OF HIGH ADHERENCE COMPRISING PLATING THE SURFACE OF THE BASE METAL WITH A LAYER CONSISTING ESSENTIALLY OF CHROMIUM TO A MAXIMUM THICKNESS OF 0.5 MIL AND DIFFUSION ANNEALING THE PLATED ARTICLE TO SUBSTANTIALLY COMPLETELY DIFFUSE THE CHROMIUM INTO THE BASE, THEREBY FORMING A CHROMIUM-STABILIZED SURFACE LAYER OF BETA TITANIUM-CHROMIUM ALLOY ON SAID BASE METAL. AN ARTICLE OF TITANIUM OR TITANIUM ALLOY BASE METAL HAVING A HARD SURFACE COMPRISING A BODY OF SAID BASE METAL AND A DIFFUSED ZONE ON THE SURFACE OF THE BASE METAL CONSISTING OF ESSENTIALLY OF A CHROMIUM-STABILIZED, BETA TITANIUM-CHROMIUM ALLOY.

Feb. 2, 1971 OGDEN 3,560,27Q

WEAR-RESISTANT TITANIUM AND TITANIUM ALLOYS AND METHOD FOR PRODUCING SAME 3 Sheets-Sheet 1 Original Filed July 29. 1966 O H 02 352E.

INVENTOR HORACE R. OGDEN mm m u 5:: $5: gwm iii iii :wm :mm w 252 M23 S10E75 2220110 $219555 592m @9113 mi: Quiz? 205 33.55 M q f w m m O n h h 4 m m O I u M U O O Q m 4 4 D o u m o A U 7 /!OIO m o 0 12: n m

oom -08 U u m 4 H N BY I Qu/wefl any"! ATTOR N EYS United States Patent Oflice Patented Feb. 2, 1971 3,560,274 WEAR-RESISTANT TITANIUM AND TITANIUM ALLOYS AND METHOD FOR PRODUCING SAME Horace R. Ogden, Columbus, Ohio, assignor to International Business Machines Corporation, Armonk, N.Y., a corporation of New York Continuation of application Ser. No. 568,903, July 29, 1966. This application Oct. 10, 1969, Ser. No. 866,151 Int. Cl. C23b 5/06, 5/52; 1332b /00 US. Cl. 14831.5 14 Claims ABSTRACT OF THE DISCLOSURE Producing a titanium or titanium alloy base metal article having a hard surface region of high adherence comprising plating the surface of the base metal with a layer consisting essentially of chromium to a maximum thickness of 0.5 mil and diffusion annealing the plated article to substantially completely diffuse the chromium into the base, thereby forming a chromium-stabilized surface layer of beta titanium-chromium alloy on said base metal.

An article of titanium or titanium alloy base metal having a hard surface comprising a body of said base rnetal and a diffused zone on the surface of the base metal consisting of essentially of a chromium-stabilized, beta titanium-chromium alloy.

This application is a continuation of application Ser. No. 568,903, filed July 29, 1966, now abandoned.

The present invention is concerned with titanium and titanium alloy articles having hard, wear-resistant surfaces. The invention is also concerned with methods for producing such articles.

It is well recognized that titanium and titanium alloys possess properties of strength and light weight which make them highly desirable for a variety of uses. For example, such metals have wide application in the space and aircraft industries and more recently are finding utility in components for high speed business machines.

Despite the basic advantages of titanium and its alloys for many purposes, their resistance to wear is notoriously inadequate for applications involving friction contact. In an attempt to overcome this problem, the metal and its alloys have been provided with various coatings of other metals to alleviate the condition of wear and galling.

.However, prior attempts to coat titanium and titanium alloys have generally been unsatisfactory for a number of reasons. In some cases, the resulting coating or surface layer did not provide hardness on the order sought. In other instances, the coating was not sufiiciently adherent to the base metal, especially where the metal was subsequently subjected to solution heat treating and aging to improve yield strength or other properties.

Accordingly, an object of the present invention is to provide titanium and titanium alloy articles having high surface hardness and improved wear resistance. A collateral object is to provide titanium and titanium alloy articles which have the desired properties and which remain substantially unimpaired by solution heat treatment and aging of the base metal.

A further object of the present invention is to provide a method for producing titanium and titanium alloy articles having high surface hardness and improved resistance to wear, even where the articles are subjected to subsequent solution heat treatment and aging.

An additional object of the invention is to provide titanium and titanium alloy articles having surface layers or zones which have improved wear resistance and remain highly adherent to the base metal regardless of subsequent impact, wear and thermal treatments.

The foregoing objects and many other highly desirable advantages are achieved in accordance with the present invention which comprises, in general, plating a titanium or titanium alloy base metal with a layer of chromium and then substantially completely diffusion annealing the chromium layer into the base metal to develop a hard, chromium-stabilized, beta titanium layer on the surface of the base metal.

Preferably, the initial chromium layer is sufliciently thin so that upon subsequent diffusion annealing, the chromium is substantially completely diffused into the titanium or titanium alloy workpiece. The diffused surface layer is then a chromium rich alloy of titanium-chromium, plus whatever other elements may be alloyed in the substrate. The surface layer of titanium-chromium thus formed contains from about 10% to 30% chromium and retains the beta structure on quenching from the beta field, as may occur during subsequent heat treatment. The chromium concentration gradually decreases from the surface inwardly of the body. This layer can be subsequently age-hardened, as in an additional aging heat treatment. A second layer of chromium-stabilized transformed beta titanium alloy underlies the surface layer and contains from about 10% to 4% of chromium, the chromium content being decreasingly graduated inwardly. A third layer of alpha plus unstable beta titanium alloy is formed under the unstable beta layer and contains from 4% to 0% chromium.

The exact conditions of plating and diffusion annealing may be varied somewhat and this will have an effect on the nature of the layers formed. Likewise, variations in the titanium base metal composition will result in different, but comparable results. In general, the titanium base metal is plated with a layer of chromium having a thickness such that it may be substantially completely diffused into the base metal by thermal treatment of practical duration and intensity. Thus, the chromium plating should have a maximum thickness of about 0.5 mil and preferably 0.3 mil or less. The thermal diffusion should be conducted for a period of up to about 24 hours at a maximum temperatures of about l900 F. The annealing temperature generally will not be less than about 16-00 F.

The invention is described below with reference to the accompanying drawings wherein:

FIG. 1 is a graph, the vertical axis of which is graduated for Knoop hardness, 10 g. load and the horizontal axis for distance from the surface of the base metal in mils. The data plotted are derived from tests of a sample prepared in accordance with the invention.

FIG. 2 is a photomicrograph of a cross-section at the surface of a titanium alloy base metal treated in accordance with the invention. The legends adjacent to the photomicrograph identifies the layers formed in the product. The tables to the right of the legends set forth estimated chromium content of the layers and hardness data at given distances from the surface of the base metal.

FIG. 3 includes a similar photomicrograph, legends and tables to those set forth in FIG. 1, but for a base metal sample treated under different conditions.

The present invention will be more fully appreciated when considered in the light of the following detailed example which is concerned with preferred hard, wearresistant titanium articles and their method of manufacture.

The present invention is applicable to titanium metal and titanium based alloys. A titanium-base alloy is generally considered to be one containing at least about 50% by weight titanium. For example, the invention may be applied to a titanium -6Al-4V alloy.

The titanium and titanium alloys of interest have potential value in the fabrication of components for high speed business machines. For example, Ti-6Al-4V alloys are light-Weight and can be heat treated to high strength levels on the order of 160-170 K.s.i. (K.s.i.=l00 p.s.i.). However, the wear resistance of titanium and its alloys is generally unsatisfactory for friction contact applications. In order to improve surface hardness in accordance with the present invention, samples of a Ti6Al-4V alloy were treated.

The samples were in the form of rods having a diameter of approximately 0.25 inch. Ti-6Al-4V alloy samples of the following composition were employed:

MATERIAL Alloy constituent: Percent by wt. Aluminum 6.3

Vanadium 4.1

Iron 0.14

Carbon 0.027

Oxygen 0.196 Nitrogen 0.016 Hydrogen 0.006 Titanium balance The sample rods may also be machined into the form of type bars or other printing elements or components of high speed business machines in which surface hardness and wear resistance of the materials are critical.

Preliminary preparation (1) The titanium alloy rods were chemically polished in a HF--HNO solution to remove disturbed metal from the ground surface. A very slight reduction in the diameter of the samples results from the chemical polishing.

(2) The rods were then alkaline degreased and rinsed in commercial alkaline cleaner and tap water.

(3) The rods were repolished in the following solution at 85 F. for twenty minutes:

NH FHF-100 g./l. H SiF (31% )-200 ml./l. HNO (70% )400 ml./l. H O (distilled)-Balance After polishing, the samples were rinsed in tap water.

(4) The rods were activated in the following solution at 120 F. with an anode current density of amp per square foot and a titanium cathode. Initially, an activation period of 40 minutes was employed, but this was found to produce a rough surface. Later, a shorter period of about 2 minutes was adopted and found to be satisfactory for a separate group of samples.

Ml./l. Glacial acetic acid 875 Hydrofluoric acid (48%) 125 After activating, the samples were rinsed in tap water.

Chromium plating The titanium alloy rods were electroplated using the chromium plating solution and conditions set forth below:

Chromic acid (CrO )--250 g./l. Sulfuric acid (H SO )--2.5 g./l. Distilled water-Balance Temperaturel30 F.

Current density-300 amp/ sq. ft. Anodes93% Pb7% Sn Plating rate-1 mil per min.

Electroplating is the chromium deposition method of choice, but other techniques producing a firmly adherent plate of the desired thickness are acceptable. Thus, vapor plating or other comparable procedure may be followed.

Diffusion annealing After the chromium plating was completed, the samples were enclosed in argon-filled Vycor capsules and individual rod samples were diffusion annealed according to the following different schedules:

The articles may be diffusion annealed by placing them in sealed capsules containing a substantially inert atmosphere and heating for the desired period. For example, the article may be placed in an Argon-filled Vycor capsule. Annealing temperatures on the order of from about .1600" to 1900 F., are preferred.

Additional samples A, B, C and D were prepared, generally in accordance with the preceding description. Chromium plate layers of 0.05 and 0.10 mil were applied. The thermal treatments of the samples after diffusion annealing also were varied.

Samples A and B, plated with a chromium thickness of 0.05 and 0.10 mil respectively, were diffusion annealed TABLE I.HARDNESS PENETRATION DATA [Ti-6A1-4V Samples, Cr plated, diffusion annealed and solution treated plug aged.]

Distance from surface, mils Sample 0 Sample D Sample A Sample B 0.05-mil Cr, 0.10-mil Cr,

0.05-mil Or, 0.10-mil Cr, Traverse Traverse Traverse Traverse Number Number Number Number Diffusion annealed and solution heat treated 3 Diffusion annealed 16 hrs.

1 Activation treatment prior to Cr plating was {01'40 minutes.

2 1,700 F., air-cooled. Solution heat-treated hour, 1,750 F., waterquenclicd. Aged for 6 hours, at 1,000 F., air-cooled.

In (i113 treatment of 16 hours, at 1,750 F., waterquenelied and aged for 6 hours at 1,000 F., air-coo e for 16 hrs. at 1700 F., air cooled, solution heat treated for /2 hr. at 1750 R, water quenched, and aged for 6 hrs. at 1000 F., followed by air cooling.

Samples C and D, plated with 0.05 and 0.10 mil of chromium respectively were diffusion annealed and solution heat treated in one treatment for 16 hrs. at 1750 F., water quenched and aged for 6 hrs. at 1000 F., followed by air cooling.

Hardness data for three traverses of each sample are set forth in Table I.

The hardness data for Sample A are presented graphically in FIG. 1. The vertical axis of the graph is graduated, for KHM, g. load, and the horizontal for distance from the surface of the sample, in mils.

It will be seen that hardness near the surface is on the order of 700 K'HN and decreases gradually to about KHN, about 9 mils from the surface. This shows a high hardness in the surface region influenced by the diffusion annealed chromium, gradually decreasing hardness in the intermediate layers, and substantially lower hardness of the base metal itself. The legends below the horizontal axis of the graph identify the metal structures formed in the sample at the depths indicated.

Generally, the articles of the invention will have a Knoop hardness, in at least a portion of the diffused, chromium-stabilized, beta zone, above 500 and usually on the order of from 600 to 700, measured under a 10 g. load.

The traveFse numbers refer to successive sections taken through the same sample rod in the course of testing hardness, after treatment.

FIG. 2 is a photomicrograph of a sample prepared by chromium plating to a thickness of about 0.05 mil and diffusion annealing for hrs. at 1700 F. The sample was then air cooled. Activation prior to chromium plating was for 2 minutes. The base metal is Ti-6Al-4V.

As will be seen from the identifying legends accompanying the photomicropgraph, the surface layer is a chromium-stabilized stable beta layer having an estimated chromium content ranging from about 30% at the surface to about 11% at a distance of 1.0 mil from the surface.

The next layer formed by the treatment is a chromiumstabilized transformed beta layer having an estimated chromium content ranging from about 11% at a distance 1.0 mil from the surface to about 4% at a distance between 30 and 4.0 mils from the surface.

The next layer is a chromium enriched alpha-beta layer having an estimated chromium content ranging from about 4% at a distance of between 3.0 and 4.0 mils from the surface and 0% at a distance approaching 5.0 mils from the surface.

A scale of the sample dimension in mils keyed to the photomicrograph is provided adjacent to the tabulated hardness and depth data. Note that high hardness was found in the surface layers extending over the zone from about 1.0 mil to 5.0 mils from the surface.

FIG. 3 is a photomicrograph and related data for another titanium article prepared in accordance with the invention. A Ti6Al-4V alloy rod, activated for 40 minutes as described above, was plated with a 0.05 mil thickness of chromium. The sample was diffusion annealed for 16 hrs. at 17 00 C., and air cooled. Legends and data identifying the layers, estimated chromium content and hardness of the sample are presented adjacent to the photomicrograph, as in FIG. 2.

The data indicates high hardness in the surface layers formed by diffusion annealing of the chromium plate and subsequent heat treatment.

What is claimed is:

1. A method for producing an article of titanium or titanium alloy base metal having a hard surface region of high adherence to the base metal comprising:

(a) electroplating at least a portion of the surface of said base metal with a layer consisting essentially of 6 chromium, said layer having a maximum thickness of 0.5 mil; and

(b) diffusion annealing the resulting plated article in a substantially inert atmosphere at a temperature of 16001900 F. for up to about 24 hours to substantially completely diffuse said chromium into said base metal and to form a high hardness, wear-resistant, chromium-stabilized surface layer of beta titanium-chromium alloy on said base metal.

2. The method of claim 1 wherein said base metal is a titanium alloy comprising by weight about 6% aluminum, 4% vanadium, and the balance substantially all titanium.

3. The method of claim 1 further comprising solution heat treating and aging said article after diffusion annealing to improve the strength thereof.

4. The method of claim 1 wherein said electroplating is conducted to a maximum thickness of 0.3 mil.

5. The method of claim 1 which comprises, prior to electroplating, chemically polishing said titanium alloy base meal, degreasing said base metal, repolishing said base metal, and then electrolytically activating said base metal to produce a roughened surface thereon.

6. An article of titanium or titanium alloy base metal having a hard surface comprising:

(a) a body of said base metal; and

(b) a diffused zone on the surface of said base metal consisting essentially of an outer surface layer of a chromium-stabilized, beta titanium-chromium alloy containing 30-10% chromium, and

a second layer of chromium-stabilized transformed beta titanium alloy thereunder containing 10- 4% chromium, and a third layer of alpha plus unstable beta titanium alloy formed thereunder containing from 4% to 0% chromium.

7. The process of claim 6 wherein said chromium content of each of said zones gradually decreases inwardly.

8. The article of claim 6, wherein the diffused zone has a depth of up to about 10 mils.

9. The article of claim 6, wherein said diffused zone comprises a portion having a Knoop hardness on the order of from 600 to 700, measured under a 10 g. load.

10. The article of claim 6, wherein said titanium base metal is a titanium alloy comprising by weight about 6% aluminum, 4% vanadium and the balance substantially all titanium.

11. The article of claim 6, wherein said outer surface layer extends to a depth of about 1.0 mil in from the surface, wherein said second layer terminates at a distance between 3.0 and 4.0 mils from the surface, and wherein said third layer terminates at a distance approaching 5.0 mils from the surface.

12. The method of claim 1 wherein said high hardness, wear-resistant, chromium-stabilized surface layer of beta titanium-chromium alloy contains 30-10% chromium, and, there is present thereunder in the following order,

a second layer of chromium-stabilized transformed beta titanium alloy containing 104% chromium, and

a third layer of alpha plus unstabled beta titanium alloy formed thereunder wherein the chromium content decreases from 4% to 0%.

13. The process of claim 1 wherein said chromium is electroplated on said base metal in a layer having a maximum thickness of from 0.3 mil to 0.5 mil.

14. The process of claim 1 wherein said diffusion annealing results in a diffused zone having a Knoop hardness on the order of from 600 to 700, measured under a 10 g. period load.

References Cited UNITED STATES PATENTS 2,645,575 7/1953 Herres et al 148l33X (Other references on following page) UNITED 7 STATES PATENTS Abkowitz et al. 148133X Vigor 29--198 Boegehold et a1 29-198X Wagner 29198X Vordahl 148133 Jefferys 29198X Gisser et a1. 29198X Handbook on Titanium, WADC Technical Report, 54-305, Part I, August 1954, pages III-4-8 and III-49.

5 CHARLES N. LOVELL, Primary Examiner U.S. Cl. X.R. 

